I think it's a combination of factors. The bottom line though is extremely poor quality control. In the case of the failed fuel pump on my 1950 Chevy, it was poor poorly constructed. The lever arm was not assembled correctly.
The new replacement that wouldn't pump a drop right out of the box is anybody's guess. I suspect even a crappy valve would pump a little bit of gas, so that's why I suspect the lever arm was either not making contact with the cam or the throw is not calibrated correctly so it doesn't move enough to activate the diaphragm.
On other modern pumps, I've had the diaphragm tear after a short period of use. Most recently on some Cadillac pumps, the valves would not stay seated in the pump because of poor quality control. Maybe the orifice was machined a couple thousands over size and the valve wouldn't seat properly, who knows?
The only fuel pump I've had in the last 20 years where I can specifically pinpoint the exact reason for the failure, was the original fuel pump on my 56 cadillac. It died within 2 months of ethanol fuel being mandated in the city of Atlanta and metro area in general. The ethanol expanded the diaphragm material to such a degree that it would only pump half the specified volume of fuel required.
